Lattice energy is the energy required to dissociate one mole of an ionic compound to its constituent ion in the gaseous state. Lattice energy ∝
Charge
Size
Greater the charge present on ion, greater will be the lattice energy while lesser the size of the ion, greater the lattice energy of compound. LiF⟶Li++F− NaCl⟶Na++Cl− MgO⟶Mg2++O2− LiCl⟶Li++Cl− Charge: Mg2+>Na+>Li+ Lattice energy: MgO > LiF > LiCl > NaCl Hence, MgO due to its greater charge present on ions have highest lattice energy.